This invention concerns thermostable enzyme breakers for the hydrolysis of galactomannans in hydraulic fracturing fluids.
When the pressure of oil or gas in a reservoir declines as oil or gas is taken from that reservoir, production from a well in that reservoir declines and the economic viability of the well declines until it is no longer profitable to operate (even though it continues to produce gas or oil). Production can be increased from such wells through oil well stimulation. In addition, where forming a bore hole into a reservoir is very expensive, such as in offshore drilling, it is desirable to stimulate production from a single well.
Oil well stimulation typically involves injecting a fracturing fluid into the well bore at extremely high pressures to create fractures in the rock formation surrounding the bore. The fractures radiate outwardly from the well bore, typically from 100 to 1000 meters, and extend the surface area from which oil or gas drains into the well. The fracturing fluid typically carries a propping agent, or xe2x80x9cproppantxe2x80x9d, such as sand, so that the fractures are propped open when the pressure on the fracturing fluid is released, and the fracture closes around the propping agent. This leaves a zone of high permeability (the propping agent trapped and compacted in the fracture in the subterranean formation.
The fracturing fluid typically contains a water soluble polymer, such a guar gum or a derivative thereof, which provides appropriate flow characteristics to the fluid and suspends the proppant particles therein. When pressure on the fracturing fluid is released and the fracture closes around the propping agent, water is forced therefrom and the water-soluble polymer forms a compacted cake. This compacted cake can prevent oil or gas flow if not removed. To solve this problem, xe2x80x9cbreakersxe2x80x9d are included in the fracturing fluid.
Currently, breakers are either enzymatic breakers or oxidative breakers. The enzyme breakers are preferable, because (a) they are true xe2x80x9ccatalystsxe2x80x9d, (b) they are relatively high in molecular weight and do not leak off into the surrounding formation, and (c) they are less susceptible to dramatic changes in activity by trace contaminants. Oxidative breakers, on the other hand, are low in molecular weight and leak off into the formation, and they are active only over a very narrow temperature range. Enzyme breakers, however, are inactive at higher temperatures, limiting their use to shallow wells. It would accordingly be highly desirable to have enzyme breakers that operate at higher temperatures to enable fracturing of deep wells. See generally J. Gulbis, Fracturing Fluid Chemistry, in RESERVOIR STIMULATION, Chap. 4 (J. J. Economides and K. G. Nolte, Eds., 2d Ed. 1989).
U.S. Pat. No. 4,996,153 to Cadmus and Slodki discloses a heat-stable enzyme breaker which may be used as a viscosity breaker in oil recovery, but this breaker is a xanthanase for degrading xanthan-based rather than guar-based fracturing fluids, and is only said to be active at 55xc2x0 C. (156.6xc2x0 F.).
U.S. Pat. No. 5,201,370 to Tjon-Joe-Pin discloses enzyme breakers for galactomannan-based fracturing fluids, which enzyme breakers are galactomannases that hydrolyze the 1,6-xcex1-D-galactomannosidic and the 1,4-xcex2-D-mannosidic linkages in the guar polymer, but these are said to only be active at low to moderate temperatures of about 50xc2x0 F. to 180xc2x0 F.
U.S. Pat. No. 4,250,044 to Hinkel concerns a tertiary amine/persulfate breaker system, and not an enzyme system.
In view of the foregoing, there is a continued need for thermostable enzyme breakers useful for fracturing subterranean formations in the course of oil and gas well stimulation.
A first aspect of the present invention is a method of fracturing a subterranean formation which surrounds a well bore. The method comprises the steps of providing a fracturing fluid, and injecting the fracturing fluid into the well bore at a pressure sufficient to form fractures in the subterranean formation which surrounds the well bore. The pressure is then released from the fracturing fluid, after which the fluid may be removed from the well and the well placed into production. The fracturing fluid comprises an aqueous liquid, a polysaccharide soluble or dispersible in the aqueous liquid in an amount sufficient to increase the viscosity of the aqueous liquid, an enzyme breaker which degrades said polysaccharide at a temperature above 180xc2x0 F.
A second aspect of the present invention is a hydraulic fracturing fluid useful for fracturing a subterranean formation which surrounds a well bore. The fracturing fluid comprises an aqueous liquid; a polysaccharide soluble or dispersible in the aqueous liquid in an amount sufficient to increase the viscosity of said aqueous liquid (said polysaccharide typically included in said aqueous liquid in an amount of from about 0.1 to 1 percent by weight); and an enzyme breaker which degrades said polysaccharide at a temperature between 180xc2x0 F. and 280xc2x0 F., the enzyme breaker included in an amount effective to degrade the polysaccharide at that temperature.
A third aspect of the present invention is an enzyme breaker useful for preparing hydraulic fracturing fluids for fracturing a subterranean formation which surrounds a well bore. The enzyme breaker comprises in combination, a mannanase which degrades polysaccharide at a temperature above 180xc2x0 F. and an xcex1-galactosidase which degrades polysaccharide at a temperature above 180xc2x0 F.
A fourth aspect of the present invention is a heat-stable xcex1-galactosidase composition which hydrolyzes xcex1-1,6 hemicellulolytic linkages in galactomannans, is isolated from hyperthermophilic organisms (e.g., as a cell-free extract thereof), is active at a temperature above 180xc2x0 F., and is essentially inactive at a temperature of 100xc2x0 F. or less.
In addition to making available enzyme breakers for higher temperature wells, a still further advantage of the present invention is that, by employing enzyme breakers which are essentially inactive at the temperature at which the fracturing fluid is initially provided, the problem of premature breaking of the fracturing fluid is inhibited or reduced. As discussed in U.S. Pat. No. 3,922,173 to Misak at columns 1-2, premature breaking of the gelled fracturing fluid (or xe2x80x9csanding outxe2x80x9d) can cause suspended proppant particles to settle out of the fracturing fluid before being introduced a sufficient distance into the fractures. This causes blockage of the fracture and/or an undesirable diminution of potential fracture width. The problem of premature breaking is reduced or inhibited in the present invention because the enzyme breaker is essentially inactive at the temperature at which the fracturing fluid is initially provided (e.g., usually ambient temperature, and not more than 90 or 100xc2x0 F.). The enzyme becomes active at the elevated temperatures encountered in the subterranean formation surrounding the well bore, which is precisely the point at which enzyme activity leading to a breaking of the fracturing fluid viscosity is desired. Thus, the present invention advantageously provides a temperature control means for controlling the timing of activation of the enzyme breaker.
E. Luthi et al., Appl. Environ. Microbiol. 57, 694 (1991), and M. Gibbs et al., Appl. Environ. Microbiol. 58, 3864 (1992), both concern a heat-stable xcex2-mannanase, but do not disclose a heat stable xcex1-galactosidase, do not suggest their use in oil well fracture fluids, and do not address the problem of premature breaking.
The foregoing and other objects and advantages of the present invention are explained in detail in the drawings herein and the specification set forth below.